Photoshop: Link layer (smart objects copies)

Layer that links to source layer. If source layer changes link layer changes too. (Direct edits to pixels of link layer are not possible, only through smart filters)

This would be usable when creating effects where only smart filters and different blending modes are applied to duplicated layer. Currently when creating effect like this, all the duplicated layers need to be updated manually, if changes to source layer are made.

Hello, it already exists:
Create a smart object for your original layer (you can still edit it by double-clicking it)
Drag that smart object to the new layer icon at the bottom of the layer panel, you have now created a child of that smart object. Every changes you make inside the smart object are reflected on the other copy.

If you right-click on a smart object and select new smart object via copy, you create an independend smart object.

»... and different blending modes are applied to duplicated layer.«
If this means you duplicate images as Layers to superimpose them onto themselves with various Blend Modes you could save on file-size and gain on non-destructivety by instead using unedited Adjustment Layers set to those Blend Modes.

I did not make the claim that Filter applications could be achieved with Adjustment Layers, but referred to the technique if superimposing copies with changed Blend Modes onto images that comes up sometimes.

Correct, you can't see the result until you save the child file and the parent updates. That's an explicit update/cache step, and part of the design of smart objects (otherwise the processing would be insanely slow).

Unfortunately using smart objects slows down the workflow, because it is so disconnected from main file. At least this is my personal experience.

Perhaps sometime in the future with more processing power/gpu acceleration, there would be an option for some alternative to smart objects that would improve non-destructive workflow. Occasionally I wish that Photoshop layers had some basic node-like functionality. It would have been really useful sometimes.

For the next version of Photoshop, I would love to see a feature that made it quick and easy to replace a different version of the same image in a file. For example, I frequently have to replace out a low-res version of an image on a layer with a higher-res. version of the same exact image. Instead of having to reduce the opacity of the top (higher-res.) layer and use the size tool to try to match it to the lower-res. version on the layer below, making it larger and smaller over and over, it would be so nice if it could just snap into place at the same exact size. If there was some automatic way to detect when the pixels matched up perfectly it would save me SO much time and frustration. It's very hard to get them to match up perfectly by hand.

I'd also love to see a way to collect all the fonts in a Photoshop file (like in InDesign) or at least see a list of them (like in Illustrator) so I don't have to go through my files and tediously click on every single font layer to see what it is, write it down and then collect it by hand to send to a printer.

Nope. It should do that automatically based on the physical dimensions of the new content. If they both say 5 inches, then the result will stay at 5 inches, regardless of resolution. So you can replace your 72dpi FPO with 600 dpi final art, and it all just works.